Online content can include web pages and advertisements displayed with the web pages. An online advertisement service can provide online advertisements on behalf of advertisers and measure statistics about the advertisements. For example, the online advertisement service might measure and record statistics about how often an advertisement displayed on web pages is clicked by a user viewing the web pages. The presence of real-world conditions may affect the way in which the user interacts with advertisements, and so the statistics may vary in accuracy depending on the presence of such conditions.